1. Field of the Invention
The invention relates to a control device for a multi-cylinder internal combustion engine and, more particularly, to a control device for a multi-cylinder internal combustion engine, which is able to carry out external EGR by which exhaust gas in an exhaust passage is circulated to an intake passage.
2. Description of Related Art
There is known a technique that is able to suppress the amount of generated NOx by carrying out external EGR to decrease the combustion speed and the combustion temperature. In addition, for example, there is also known a technique that, in an internal combustion engine equipped with a catalyst, in order to purify toxic substances in exhaust gas by the catalyst with high efficiency, air-fuel ratio feedback control that brings the air-fuel ratio of air-fuel mixture and, by extension, the air-fuel ratio of exhaust gas, close to a predetermined target air-fuel ratio.
Incidentally, in a multi-cylinder internal combustion engine, for example, the fuel injection systems of part of the cylinders fail and, as a result, the air-fuel ratios may significantly vary among the cylinders. Such a significant air-fuel ratio variation remarkably deteriorates exhaust gas emissions, so such a variation is desirably detected as an abnormality.
When an abnormal air-fuel ratio variation occurs while air-fuel ratio feedback control is being executed and external EGR is being carried out, cylinders that contain an air-fuel mixture having an air-fuel ratio richer than a target air-fuel ratio (rich cylinders) and cylinders that contain an air-fuel mixture having an air-fuel ratio leaner than the target air-fuel ratio (lean cylinders) are mixed. Then combustion deteriorates in the lean cylinders in combination with external EGR, so relatively large amounts of unburned components, such as H2, CO and HC, are emitted.
On the other hand, air-fuel ratio feedback control is configured to adjust the air-fuel ratio detected by an air-fuel ratio sensor to a target air-fuel ratio. The air-fuel ratio detected by the air-fuel ratio sensor tends to deviate from a true value because of the influence of the above unburned components (particularly, hydrogen H2), and it is conceivable to carry out correction for compensating for the deviation, that is, an error.
However, the intensity of gas that flows (gas flow intensity) against the air-fuel ratio sensor is different among the cylinders. Therefore, when correction is carried out uniformly without taking the difference into consideration, the correction may be inappropriate.